Printing apparatuses are known in the art, which can hold a roll of printing medium such as a label sheet. In the prior art, a printing apparatus includes a housing and a sheet holder provided in the housing to hold a roll of printing medium. The sheet holder has two first support units, two rack gears, two pinion gears, and a control unit. The first support units are arranged at the sides of the roll in the axial direction of the roll, respectively, and can be moved in the axial direction. The rack gears are provided on the two first support units, respectively, and the pinion gears mesh with the two rack gears, respectively. The rack gears and the pinions cooperate to move the two first support units. The control unit sets a protrusion between any two comb teeth provided in the housing, thereby controlling the motion of the two first support units in the axial direction. The sheet holder has two first holding parts at the ends spaced in the width direction of the rolled printing medium, and holds the roll in a rotatable state, while the control unit is controlling the motion of the two first support units.